1. Field of the Invention
The present invention relates to communication systems for communicating a large amount of data between information devices, and more particularly to a communication system for performing high-capacity data communication between information devices using an ultra-wideband (UWB) communication scheme using a radio-frequency (RF), wideband signal.
More specifically, the present invention relates to a communication system, a communication apparatus, and an electric-field-coupling antenna (hereinafter called “EFC antenna”) for transmitting a UWB communication signal between information devices disposed at a very short distance using an electrostatic field (quasi-electrostatic field) or an induced electric: field, and more particularly to a communication system, a communication apparatus, and an EFC antenna for enabling high-capacity transmission by efficiently transmitting an RF signal between EFC antennas of information devices using an electrostatic field or an induced electric field over a very short distance.
2. Description of the Related Art
Recently, the use of wireless interfaces in transferring data between small information devices, such as when image data or music data is exchanged between a small information device and a personal computer (PC), is becoming more widespread in place of data transmission using a general cable, such as an audio/visual (AV) cable or a universal serial bus (USB) cable, to interconnect the information devices or using a medium such as a memory card. Wireless interfaces are user-friendlier because they do not involve the reconnection of connectors and the wiring of a cable every time data transmission is performed. Various information devices having cableless communication functions are now available.
As methods of cableless data transmission between small devices, besides wireless local area networks (LANs) represented by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and Bluetooth®, radio communication schemes of performing transmission/reception of radio signals using antennas have been developed. For example, a proposal has been made for a portable image recording apparatus containing an internal antenna placed at a position not covered with a hand holding a grip (for example, see Japanese Unexamined Patent Application Publication No. 2006-106612). Since the internal antenna is not covered with the hand, correct image data is received. Even in the case where the antenna for wireless communication is placed inside the apparatus, the antenna can exert its characteristics.
A communication scheme referred to as “ultra-wideband” (UWB), which has been drawing increasing attention in recent years, is a wireless communication technology for implementing high-capacity wireless data transmission at approximately 100 Mbs, though over a short distance, using a very wide frequency band from 3.1 GHz to 10.6 GHz, Therefore, a large amount of data, such as a moving image or music data contained in one compact disc (CD), can be transferred at high speed in a short period of time.
The UWB communication has a communication distance of approximately 10 m due to its relationship with transmission power. Using UWB, a short-distance wireless communication scheme, such as a personal area network (PAN), may be configured. For example, in IEEE 802.15.3 or the like, the scheme of transmitting data having a packet structure including a preamble is designed as an access control scheme in the UWB communication. Intel Corporation in the United States is studying, as a UWB application, a wireless version of USB, which has been widely used as a general interface for PCs.
UWB transmission systems using a UWB low-band from 3.1 GHz to 4.9 GHz have been actively developed taking into consideration that the UWB communication can transfer data at a rate exceeding 100 Mbps without occupying a transmission band from 3.1 GHz to 10.6 GHz, and the simplicity of fabricating an RF circuit. The inventor of the present invention believes that a data transmission system employing a UWB low-band is one of effective wireless communication techniques to be applied to mobile devices. For example, high-speed data transmission in a short-distance area, such as an ultra-high-speed, short-distance device area network (DAN) including a storage device, can be implemented.
In the case where the strength of an electric field (the strength of radio waves) at a distance of 3 m from a wireless facility is less than or equal to a predetermined level, that is, in the case where very weak radio waves merely causing noise for other wireless systems existing in the neighborhood are emitted, the licensing of a radio station is unnecessary (for example, see the Regulations for the Enforcement of the Radio Law, Article 6(1-1) (Regulation No. 14, Radio Regulatory Committee, 1950)). Therefore, the development and production cost of a wireless system can be reduced. Due to its relationship with transmission power, the above-described UWB communication can implement a short-distance wireless communication system at a relatively low electric-field level. However, in the case of a UWB communication system using a radio communication scheme for performing transmission and reception of radio signals using antennas, an electric field generated therein is difficult to be suppressed to a very weak level.
Many known wireless communication systems employ radio communication schemes and allow signals to propagate using a radiated electric field generated in the case where current flows through an aerial (antenna). In this case, a transmitter side emits radio waves regardless of the presence of a communication partner. This may generate jamming waves which disturb the neighboring communication systems. An antenna at a receiver side receives not only desired waves from the transmitter, but also radio waves arriving from far away. Thus, the antenna at the receiver side is susceptible to the influence of ambient jamming waves, resulting in a reduction of the reception sensitivity. In the case where a plurality of communication partners exists, a complicated setting is necessary to be done in order to select a desired communication partner from the plurality of communication partners. For example, in the case where a plurality of pairs of wireless devices perform wireless communication within a narrow range, division multiplexing such as frequency selection is necessary to be done in order to perform communication that does not interfere with other pairs of wireless devices. Since a radio wave perpendicular to the direction of polarization is not communicated, the directions of polarization of the antennas of the transmitter and the receiver are necessary to be aligned.
For example, in the case of a contact less data communication system in which communication is performed within a very short distance from a few millimeters to a few centimeters, it is preferable that the transmitter and the receiver be coupled strongly within a short distance, but a signal not reach across a long distance in order to avoid interference with other systems. It is also preferable that data communication devices be coupled with each other regardless of the orientations (directions) thereof in the case where the data communication devices are placed in close proximity with each other, that is, it is preferable that the devices have no directivity. In order to perform high-capacity data communication, it is preferable that the devices can perform broadband communication.
In wireless communication, besides the above-mentioned radio communication using a radiated electric field, various communication schemes using an electrostatic field or an induced electric field are available. For example, an electric-field coupling scheme or an electromagnetic-induction scheme is employed in existing contactless communication systems mainly used in radio frequency identification (RFID). An electrostatic field or an induced electric field is inversely proportional to the cube or the square of the distance from the source thereof. Therefore, very weak radio waves in which the strength of an electric field (strength of radio waves) is less than or equal to a predetermined level at a distance of 3 m from a wireless facility can be emitted, and hence the licensing of a radio station is unnecessary. In this type of contactless communication system, a transmission signal is rapidly attenuated according to the distance. In the case where no nearby communication partner exists, no coupling relationship occurs, and hence other communication systems are not disturbed. Even in the ease where radio waves arrive from far away, an EFC antenna receives no radio waves, and hence the present system is not interfered with by other communication systems. That is, contactless, very-short-distance communication using electric field coupling employing an induced electric field or an electrostatic field is suitable for emitting very weak radio waves.
Contactless, very-short-distance communication systems have some advantages over general wireless communication systems. For example, in the case where a radio signal is exchanged between devices located at a relatively long distance, the quality of the radio signal in a radio range is deteriorated due to a neighboring reflector or an increase in the communication distance. In contrast, short-distance communication is not dependent on its surrounding environment and can transfer a high-quality signal with a low error rate at a high transfer rate. In a very-short-distance communication system, there is no chance for an unauthorized device to intercept transmission data. Therefore, it is unnecessary to take into consideration prevention of hacking on a transmission line and securing of confidentiality.
In radio communication, it is necessary for an antenna to have a size of approximately one-half or one-fourth of its operating wavelength λ, which leads to an increase in the size of an apparatus. In contrast, there are no such limitations in a very-short-distance communication system using an induced electromagnetic field or an electrostatic magnetic field.
For example, a proposal for an RFID tag system has been made (for example, see Japanese Unexamined Patent Application Publication No. 2006-60283). In this system, pairs of communication auxiliary units are arranged so that RFID tags are positioned between a plurality of communication auxiliary units. By arranging RFID tags attached to a plurality of merchandise items so as to be sandwiched between the communication auxiliary units, stable reading and writing of information can be implemented even in the case where RFID tags overlap one another.
A proposal for a data communication apparatus using an induced magnetic field has been made (for example, see Japanese Unexamined Patent Application Publication No. 2004-214879). The apparatus includes a main body, a mounting tool configured to mount the main body to a physical body of a user, an antenna coil, and a data communication unit configured to perform contactless data communication with an external communication apparatus via the antenna coil. The antenna coil and the data communication unit are placed on an outer casing provided above the main body of the apparatus.
A proposal has been made for a cellular phone, which is en exemplary portable information device, provided with an RFID, which ensures a communication distance without sacrificing the portability (for example, see Japanese Unexamined Patent Application Publication No. 2005-18671). A memory card to be inserted into the portable information device includes an antenna coil configured to perform data communication with an external device. An RFID antenna coil is provided in the exterior of a memory-card insertion slot of the portable information device.
Because known RFID systems employing an electrostatic field or an induced electric field use lower-frequency signals, the systems communicate at a low speed and are thus not suitable for high-capacity data transmission. In the case of communication scheme using an electromagnetic field induced by an antenna coil, no communication can be performed in the case where a metal plate is positioned behind the coil. A large planar area is necessary for mounting the coil. Such mounting problems are found in this type of communication scheme. Further, a transmission line has a large loss, and hence the signal transmission efficiency is poor.
In contrast, the inventor of the present invention believes that high-speed data transmission taking into consideration confidentiality can be implemented by emitting a very weak electric field which does not call for the licensing of a wireless station in a very-short-distance communication system for transmitting an RF signal using electric-field coupling, that is, transmitting the above-mentioned UWB communication signal by employing an electrostatic field or an induced electromagnetic field. The inventor of the present invention believes that, in the UWB communication system using an electrostatic field or an induced electric field, a large amount of data, such as a moving image or music data contained in one CD, can be transferred at high speed in a short period of time.
In known RFID systems, electrodes (EFC antennas) of a transmitter and a receiver are generally attached to each other, which is inconvenient for a user. It is thus desirable to perform short-distance communication in which, the electrodes are separated by a distance of approximately 3 cm.
In an electric-field coupling scheme using signals in a relatively low frequency band, the 3-cm distance between the electrodes of the transmitter and the receiver is negligible relative to the wavelength, and the propagation loss between the transmitter and the receiver is not a serious problem. However, when transmitting a higher-frequency, wideband signal such as a UWB signal, a distance of 3 cm corresponds to approximately one-half of the wavelength of an operating frequency of 4 GHz. Since the propagation loss occurs in accordance with the propagation distance relative to the wavelength, the distance between the electrodes of the transmitter and the receiver is considerably long relative to the wavelength. Therefore, the propagation loss should be sufficiently suppressed in the case where a UWB signal is transmitted using electric-field coupling.